First of all, each problem in Exercism has a set of unit tests, so you don’t have to wonder if you’re doing it right, you can just run the tests and know for sure. The tests are also great for experimenting with your code and seeing if you can make it easier to read or easier to change without breaking it.

The problems are also carefully chosen to help you learn concepts that are important to each language. Just because you can solve a problem in a certain language on a coding challenge site like HackerRank doesn’t mean you’re learning anything interesting about that language in particular. I know that’s not really what challenge sites are meant for, but I’ve seen them recommended to a lot of people who are learning and think it’s important to be clear about what challenge sites are good for (general programming concepts) and what they aren’t necessarily good for (learning individual languages).

And finally, Exercism directly incorporates both giving and receiving feedback. Obviously getting feedback is helpful – to directly quote the Exercism site: “You can write FORTRAN in any language, as the saying famously goes, but with enough feedback, you’ll quickly find yourself writing the language the way it wants to be written.” – but giving feedback is seriously underrated. To tell someone what you think of their code, you have to read it carefully and then think seriously about what makes code good or bad. That’s enormously helpful when you’re new and don’t really understand what “good code” means yet, or when you’re new to a language and just don’t know the best way to do things in that particular language.

Hey java devs, somebody built a REPL! It even runs in the browser, nothing to install or configure (or curse at). I love this thing for when I can’t remember which function accepts a null parameter and which one freaks out and throws a null pointer exception, it’s so helpful.

There is a REPL in Java 9, but we haven’t even switched to 8 yet at work (always let someone else try the new stuff first :) ). If you’re not exactly on the cutting edge either, try the online REPL, it’s awesome.

Considering how many javascript and other fiddles there are out there, it shouldn’t come as any sort of surprise that there’s a SQL Fiddle too. We use MongoDB at my work so I haven’t actually tried SQL Fiddle myself, but if you do write SQL and ever need help, it looks like an amazing tool for collaboration.